1. Field of the Invention
The invention relates to electron-optical systems such as electron microscopes and microanalyzers. It relates particularly to high resolution scanning electron microscopes, high resolution scanning transmission electron microscopes, and to high resolution fixed-beam transmission electron microscopes.
2. Description of Prior Art
Electron-optical systems able to form electron beams that are narrow in spatial extent and also narrow in their energy spread are able to probe matter on an atomic scale, and are able to provide valuable information about the structure of matter, its chemical composition, chemical bonding, and electronic and vibrational properties. Information about chemical composition and bonding, and the electronic and vibrational properties of matter is typically derived from electron energy loss spectra, formed by passing a monochromatic beam of electrons through a thin sample in which the beam suffers discrete energy losses, dispersing the beam in energy, and recording a spectrum of the losses. The spatial resolution of such studies is largely determined by the brightness of the electron source and by the aberrations of the electron-optical system that forms the electron probe. The energy resolution of the spectrum is largely determined by the energy width of the electron beam incident on the sample, and by the energy resolution of the spectrometer.
The spatial resolution determines the size of the smallest sample features that may be imaged and analyzed, and atomic resolution becomes possible if the electron probe is smaller than one atom. The energy resolution of the recorded energy-loss spectrum determines what kinds of information may be obtained from it. Chemical composition information may be obtained from spectra showing an energy resolution of a few eV, chemical bonding and electronic properties typically require spectra with energy resolution of a few tenths of eV, and the sample vibrational properties may be probed by spectra with an energy resolution of a few meV. Because more information becomes available at higher resolution, there is much interest in improving the ability of electron-optical systems to focus electron beams into very narrow probes, and to form energy loss spectra with very high energy resolution.
Recent introduction of correctors of geometric aberrations has allowed the spatial extent of electron beams of medium primary energies of around 100 keV to be made smaller than the diameter of a hydrogen atom (i.e., <1 Å). Cold field emission electron guns are able to produce electron beams as narrow in energy as 0.25 eV, and electron monochromators are able to produce beams of electrons with primary energy of 100-200 keV that are as narrow as 30 meV and therefore can potentially lead to 30 meV energy resolution in electron energy loss spectra. However, no electron beam apparatus has yet been able to produce an electron beam of atomic size (1 Å diameter) at a primary beam energy of less than 50 keV, nor to illuminate a sample with an atom-sized electron beam and at the same time to resolve features in electron energy loss that are separated by less than 30 meV.
Being able to produce atomic-sized probes at the lowered primary energies would be advantageous when exploring materials made from light atoms, in which a lower primary energy often results in a dramatic reduction of knock-on radiation damage. Being able to produce electron energy loss spectra with an energy resolution better than 30 meV and preferably just 1-5 meV would allow an exploration of vibrational spectra of materials, and in this way provide a particularly rich new source of information.
The principal limit on the spatial resolution in electron beam systems corrected for geometric aberrations comes from chromatic aberration. There are two principal ways how the chromatic limit may be improved. In the first way, the energy spread of the beam is decreased through the use of an optical system that disperses the electrons according to their energies, and then intercepts, typically using an energy-selecting slit, all electrons whose energies lie outside a given pass-band of energies. Such an apparatus is typically called an electron monochromator. The resultant beam of narrower energy width is then affected by the chromatic aberration of the optical system much less, and an improvement in spatial resolution is typically obtained. The monochromator also significantly improves the attainable resolution in electron energy loss spectra. However, the improvement comes at the cost of a decreased brightness of the electron beam, caused by a large part of the beam being stopped by the energy-selecting slit. In the second way, the chromatic aberration of the optical system is corrected by a corrector of chromatic aberration. The chromatic aberration corrector does not filter out any electrons and therefore preserves the brightness of the electron beam, but it cannot improve the energy resolution of electron spectra.
Many types of both electron monochromators and chromatic aberration correctors have been developed over the years, as described for instance in U.S. Pat. Nos. 5,838,004, 6,407,384 B1, 6,580,073 B2 and 6,770,878 B2. Because chromatic aberration and a lack of a sufficiently bright electron beam are the two main factors that determine the spatial resolution when operating at lowered primary energies of the order of 20-60 kV, the chromatic correction is especially suitable for this type of operation.
A useful measure of the quality of a monochromator is the order of aberrations that it is able to correct. All monochromators are able to focus electron beams as needed to first order and some are also able to do second-order focusing, but no monochromator has yet achieved full third order focusing by correcting all important third-order aberrations. The higher the order of focusing a monochromator is able to do, the greater the range of angles of the electron beam it is able to monochromate correctly. Increasing the acceptable range of angles allows beams of greater total electron current to be monochromated. The interception of electrons of unwanted energies by the monochromator's energy-selecting slit results in a significant loss of total beam current, and being able to monochromate electron beams of larger starting current is therefore a significant advantage.
Another useful distinction is that existing monochromator designs can be separated into two broad classes: monochromators that disperse the electron beam according to energy and send the dispersed beam back into the overall electron-optical apparatus without nulling the dispersion, and monochromators that disperse the beam in order to do the energy selection, and subsequently null the dispersion before re-inserting the beam into the rest of the electron-optical apparatus. The second class is often called dispersing-undispersing monochromators. The first class of monochromators is simpler to build, but suffers from the disadvantage that the uncancelled energy dispersion causes a significant broadening of the virtual source of electrons, and hence a significant loss of brightness of the incident electron beam even when the energy-selecting slit is wide open, and no energy selection is taking place.
Monochromators developed up to the present time have typically acted on an electron beam of a low energy of the order of a few hundred eV to a few keV. This increases the energy dispersion the monochromators are able to achieve, and in this way allows the selection of narrower pass-bands of energies. The electrons are typically accelerated up to their final energy only after the energy selection has been accomplished. Unfortunately, performing the energy selection on particle beams of low energy increases the importance of Coulomb interactions between the individual electrons that constitute the electron beam. This interaction then limits the ultimate spatial and energy resolution that can be attained by the apparatus. Another limitation of monochromators performing the energy selection on a low energy beam is that variations in the high voltage used for the final acceleration cannot be readily compensated in the spectrometer part of the total apparatus, unless the electron beam is decelerated before the spectrometer, which is impractical and costly, and leads to further Coulomb interactions. Most spectrometers currently in existence therefore do not decelerate the electron beam before the spectrometer, with the result that instabilities in the accelerating voltage show up as instabilities in the energy of the final energy spectrum formed by a total apparatus comprising an electron spectrometer in addition to the electron source and the monochromator.
A monochromator system described in U.S. Pat. No. 5,097,126 was designed to perform energy selection on electrons of the full primary energy, in order to decrease the deleterious effect of Coulomb interaction. Because it operated on electrons of the full primary energy, it was able to link the energy being selected by the monochromator to the energy being analyzed by a spectrometer situated downstream in the optical system, simply by running the same current in the windings of all the magnetic prisms used in the monochromator and the spectrometer that were connected in series. This should have been able to give a very useful improvement in the energy stability and the energy resolution of the total optical system. Unfortunately, this monochromator was of the type that does not cancel dispersion before re-inserting the beam back into the electron-optical column, and the maximum attainable brightness of the electron beam monochromated by it was there therefore much more limited than in a dispersing-undispersing design. Further, the monochromator was only able to do a partial correction of second order aberrations, and provided no correction of third order aberrations, and no chromatic correction. For all these reasons, the intensity of the monochromated electron beam produced by this monochromator would have been very weak if the energy-selecting slit were closed down as necessary for an energy resolution of a few meV, rendering the monochromator unsuitable for this application.
Correctors of chromatic aberration of electron-optical systems have traditionally employed crossed electrostatic and electromagnetic fields in electrostatic/electromagnetic quadrupoles or Wien filters, as for instance described in U.S. Pat. Nos. 4,962,313 and 6,797,962 B1. The electrostatic elements create difficulties due to their need for relatively high voltage (several kV) to be supplied to small electrodes held in vacuum, which often leads to discharges. Another problem that arises with this type of chromatic aberration correctors is that voltage stabilities of the order of 1 part in 108 need to be achieved if an improvement in the spatial resolution is to be reached in modern, highly-perfected electron-optical systems, and such stabilities are difficult to attain and hold for an extended period of time.
Most users of electron microscopes and microanalyzers would find significant advantage in a energy-selecting apparatus and method acting on electrons of the full primary energy and therefore able to link the energy being selected by the apparatus to the energy being analyzed by a spectrometer, and which also performed correction of geometric and chromatic aberrations. They would especially appreciate it if the chromatic correction was performed electromagnetically, and thus avoided the need to bring large voltages inside the vacuum of the energy-selecting apparatus. They would also appreciate an energy-selecting apparatus able to cancel the energy dispersion before reinserting the electron beam into the column, which was therefore better able to preserve the brightness and intensity of the electron beam. They would further very much appreciate an ability to form electron energy spectra with an energy resolution sufficient for studying the vibrational modes of materials, at the same time as producing an electron probe of atomic dimensions, so that the vibrational properties could be explored on an atomic scale.